Recently, it has come to develop a white LED having characteristics such as compactness, long life, low-voltage drive, mercury free and the like in contract to a fluorescent tube (FL) and a cold cathode fluorescent lamp (CCFL) which are excited by mercury gas and used for general lighting, a backlight of the liquid crystal display device, and the like.
The white LED includes type 1 that combines three-color light emitting diodes such as a red light emitting diode, a green light emitting diode and a blue light emitting diode to emit white light, and type 2 that combines a light emitting diode having long-wavelength ultraviolet light (300 to 430 nm) or a blue wavelength (460 to 480 nm) as an excitation source with a phosphor layer which contains a plurality of types of visible light emitting phosphors to emit white light.
Since the type 1 uses a three-color light source, a mechanism for color mixing, such as use of a diffuser plate for producing white light or provision of a space for diffusion, is required, so that a thickness is needed for the backlight. And, since three types of LEDs are used, the number of parts increases because separate control circuits are required for them, and there is a problem in view of the cost.
Since the type 2 can use the same LED for all and emits the same color by them, a mechanism for mixing colors is not needed, and the control circuit is only one type, so that the number of parts can be reduced, and it can be produced inexpensively in view of the cost. The white LED of the type 2 includes one using the long-wavelength ultraviolet (or violet) light emitting diode as described above and one using the light emitting diode which emits blue light (460 to 480 nm).
In a case where an ultraviolet (or violet) light emitting diode is used, visible light emitting phosphors of three colors of red, green and blue are used as the phosphor layer to obtain white light. On the other hand, the blue light emitting diode generally obtains the white light by using visible light emitting phosphors of two colors of red and yellow. It has come to know that when the ultraviolet light emitting diode is used, color reproducibility is better because three color phosphors are used in comparison with the blue light emitting diode which uses two colors to obtain white light.
Different from conventional excitation by mercury gas at 254 nm, the type 2 white LED is excited with long-wavelength ultraviolet light (300 to 430 nm) or a blue wavelength (460 to 480 nm) and, therefore, cannot often use the phosphor which has been used for the present FL and CCFL.
Especially, LaPO4:Ce, Tb which is generally used as a green phosphor for the FL and CCFL hardly emits light at 320 to 430 nm. Therefore, it is well known that as a green component, BaMgAl10O17:Eu, Mn is used (component satisfying the general formula), but there was a problem that the color rendering properties were deteriorated when used for lighting because as the green component, an emission wavelength is 515 nm, which is a short wavelength in comparison with 543 nm of the conventional material LaPO4:Ce, Tb, and doesn't become complete interchangeable.
On the other hand, the liquid crystal display device has its color reproducibility range determined as a display device depending on a combination of the backlight and the color filter. For example, even when the CCFL is merely replaced by the white LED in a liquid crystal display device which has a backlight using CCFL mounted, the properties of the liquid crystal display device are not improved. It is because the color filter is designed in conformity with the color reproduction region of the light of the CCFL. In other words, mere replacement of the light source from the CCFL to the white LED does not necessarily improve the properties of the liquid crystal display device.
For example, a certain reference discloses a white LED using YBO3:Ce3+, Tb3+ phosphors (cerium- and terbium-activated yttrium borate phosphors) as a green phosphor, and Y2O2S:Eu3+, Bi3+ phosphors (europium- and bismuth-activated yttrium oxysulfide phosphors) as a red phosphor. But, since a blue light emitting diode is used in the reference, the color reproduction region of the blue portion is particularly fixed by the light emitting property of the blue light emitting diode. Therefore, since the color reproduction region is different from the CCFL, it is necessary to change the design of the color filter to use the white LED of the reference for the liquid crystal display device using the conventional CCFL.
Therefore, there were demands for the white LED having the same color reproduction region as that of the conventional FL or CCFL by using the light emitting diode which has good color reproducibility, namely which can adjust the color reproduction region in a large range and emits light with long-wavelength ultraviolet light (300 to 430 nm). And, in response to the above demands, there is disclosed a white LED which is characterized by three color phosphors of red, green and blue to be combined with an ultraviolet (or violet) light emitting element.
Recent liquid crystal display devices are used for various types of screens (monitors) such as small screens of cellular phones, car navigation systems and mobile telecommunications equipment, middle- and large-size screens of personal computers and TVs, and CCFL is mainly used for their backlight light source.
To use the CCFL as the light source, there are proposed conventionally various types of optical members such as color filters and the like to make the light from the CCFL better and to improve the color reproducibility. Considering the characteristics and the like of the emitted light of the CCFL, such a color filter is designed to provide the best display quality, color reproducibility and the like in specific uses, and there are also provided many satisfactory color filters for the CCFL at present.
Recently, attempts have been made to use the LED instead of the CCFL as the light source in response to increasing demands for mercury free in view of environmental problems, but since the conventional white LED is largely different in emission color from the CCFL, mere replacement of the CCFL of the liquid crystal display device with the white LED cannot provide good display quality and color reproducibility, and the color filter designed to be suitable for the CCFL cannot be used.
Under the above circumstances, to use a conventional white LED as a light source, the color filter was also required to be redesigned so as to be suitable for the emitted light characteristics or a color reproduction region of the white LED. It is needless to say that significant cost and time are needed to develop a technology to design a new color filter and to produce it stably. Thus, it is demanded to develop a white LED that a conventional CCFL color filter can be used as it is.